1. Field of the Invention
The present invention relates to a resonant converting circuit and a resonant controller, and more particularly relates to a resonant converting circuit and a resonant controller with over current protecting function.
2. Description of Related Art
Under the global trend of energy-saving and carbon reduction in the current stage, many countries have legislated energy efficiency regulations or announced energy efficiency certifications. Therefore, the manufacturers devote to increase the conversion efficiency of power system recently. A resonant converting circuit has an advantage of zero voltage switching (ZVS) to substantially decrease the switching loss of transistors and so the resonant converting circuit is applied to more power source systems.
One of important issues of the resonant converting circuit is the over current protection. The over current protection has to be executed immediately and exactly when the resonant converting circuit operates normally, and further is capable of determining false over current conditions, for example: the resonant converting circuit is starting. FIG. 1 is a schematic diagram of a conventional half-bridge LLC resonant converting circuit. A resonant circuit comprises two transistor switches M1 and M2, a resonant capacitance Cr, a resonant inductance Lr, a transformer T (herein, only a primary side of the transformer T is shown), and a resonant controller 10. A detecting resistance Rcs is coupled to the primary side of the transformer T and generates a signal according to a resonant current of the resonant circuit, which is feedback to a current detecting terminal OC of the resonant controller 10 after being filtered by a resistance R1 and a capacitance C1. The resonant controller 10 executes over current protection when a voltage across the capacitance C1 exceeds a set threshold voltage. The circuit design of the over current protection has advantages of simple circuit structure and periodically executing over current protection. However, the aforementioned circuit structure can not differentiate false over current conditions and further the detecting of the detecting resistance Rcs causes power consumption.
FIG. 2 is a schematic diagram of another conventional half-bridge LLC resonant converting circuit. Compared to the half-bridge LLC resonant converting circuit shown in FIG. 1, the main difference is that a detecting capacitance Cs is substituted for the detecting resistance Rcs to detect the resonant current to decrease the power consumption of detecting current. A resistance R4 and a set resistance R5 have functions of voltage-dividing and current limiting, and a filter capacitance C3 has a filtering function. Diodes D1 and D2 have functions of rectification and voltage clamping. A discharge period of the filter capacitance C3 can be set by adjusting a resistance value of the set resistance R5 and a capacitance value of the filter capacitance C3, so as to set a delay time for restarting when the over current condition occurs. In FIG. 2, the current detecting is average current detecting by the filter capacitance C3. Therefore, the false over current state may be filtered by the filter capacitance C3. Nevertheless, the capacitance value of the filter capacitance C3 and the resistance values of the resistance R4 and the set resistance R5 cannot be too large due to that the large capacitance and resistance values cause response time of over current protection to be long, even fail. On the other hand, the capacitance value of the filter capacitance C3 and the resistance values of the resistance R4 and the set resistance R5 are smaller, the delay time for restarting is too short to sufficiently release an energy stored in the resonant circuit.
By the foregoing descriptions, all of the conventional over current protection cannot provide a perfect over current protection for the resonant converting circuit.